A method is known for producing the secondary linkage magnetic flux of an induction motor by using a primary delay circuit with the voltage and current on the motor and a magnetic flux command value as inputs.
For example, in Japanese Laid-Open Patent Publication No. 57-201870 previously disclosed by the present applicant, a method has been employed in which a secondary induced voltage is obtained by reducing the impedance voltage of an induction motor from the terminal voltage thereof, a voltage obtained by adding to the secondary induced voltage a voltage corresponding to the excitation command current producing by an excitation command is derived and the derived voltage is integrated by a primary delay integration.
However, the above conventional method has had the disadvantage that the effect of the parameters of the motor on the magnetic flux operation value, especially that of the primary resistance varying depending on temperatures, becomes large at low-frequencies.